1. Field of the Invention
The invention relates to an apparatus for the transmission of data between a number of interconnection units. Data receiving and/or data transmitting apparatus are connected to the interconnection units. Each interconnection unit comprises at least two physically separate data connections. Each data connection comprises at least two connection lines corresponding to respective transmission paths of opposite directions. The interconnection units are interconnected via the transmission paths so that in a non-disturbed state at least one data transmission path is formed which successively passes through all interconnection units in the apparatus. The apparatus includes a synchronization device which generates synchronization information in order to form a continual data stream. Each interconnection unit comprises a detector device for monitoring the data stream on the data transmission path. Upon detection of a disturbance of the data stream, the detector device interconnects the data connections so that there is again formed a transmission path which interconnects the interconnection units, avoiding the location of disturbance.
2. The Prior Art
Such an apparatus is known from U.S. Pat. No. 4,109,821. Therein, a number of interconnection units and a central station for generating synchronization information are connected so as to form a closed loop. Each interconnection unit comprises two data connections for an incoming line and an outgoing line. These lines between the data connections interconnect the interconnection units in such a manner that two separate, closed data transmission paths are obtained in which the direction of transmission is opposed. Only one of the two data transmission paths is used for data transmission, when all connections between all interconnection units are non-disturbed. The other data transmission path serves as a spare for disturbances. Data transmission from a first arbitrary interconnection unit to a second arbitrary interconnection unit takes place when the first interconnection unit inserts a message in a free location in the data stream. The message contains the address of the second interconnection unit. All interconnection units constantly monitor the data stream checking for their own address. If their address is found, the relevant message is taken up and the data stream is transmitted further, preferably without the relevant message. In an extreme case, an interconnection unit can thus transmit a message to the directly preceding interconnection unit in the data stream. In such a case a delay occurs because the message traverses substantially the entire data transmission path. The messages are generally transmitted in a frame structure which is fixed in time and which is determined by the synchronization information. Therefore delay time compensation in the prior art is performed by means of buffer memories, notably in the central station. This compensation is made in order to ensure that the shortest delay time, via the complete transmission path, amounts to at least one frame period.
A disturbance may occur in the data transmission path used for the data transmission due to an interruption of the connections between the interconnection units or a disturbance in the data transmission path inside an interconnection unit. Such a disturbance will be noticed by the next interconnection unit when data no longer arrive and especially when there is no synchronization information within given, fixed time intervals. This next interconnection unit then interrupts the unused data transmission path and connects the input thereof to the output for the original data transmission path. The non-disturbed state of the path which is not used for data transmission is also constantly monitored by constant transmission of monitoring data. The interconnection unit preceding the disturbed location detects the absence of this monitoring information and connects the input of the data transmission path used thus far to the output of the other data transmission path. The disturbed location is thus bridged by the other data transmission path. The delay times of transmitted messages are thus approximately doubled, but transmission of messages from a first arbitrary interconnection unit to a second arbitrary interconnection unit outside the disturbed location is still possible.
When a further disturbance occurs in another location in the data transmission path, the overall loop of interconnection units break down into two sections. A reconfiguration of the section comprising the central station for the synchronization information is then similarly possible. However, in the remainder of the interconnection units data transmission is then no longer possible. Moreover, in the section in which reconfiguration has taken place the delay time compensation must be adapted to the possible shortening of the transmission path.